An integrated processing system for harvesting microalgae using recyclable magnetic nanoparticles while simultaneously promoting methane production.

Abstract

This invention provides an integrated treatment system for harvesting microalgae using recyclable magnetic nanoparticles while simultaneously promoting methane production, belonging to the field of algal resource utilization and disposal. The specific method includes: uniformly mixing synthesized magnetic nanoparticles with algal liquid; harvesting and concentrating microalgae using magnetic separation; mixing the magnetic-algae aggregates with waste activated sludge in a certain proportion and then anaerobically treating the sludge; and finally recovering the magnetic nanoparticles from the anaerobic co-digestion residue using air flotation. Results show that compared with existing technologies, this invention achieves over 95% high-efficiency integrated treatment of microalgae harvesting and anaerobic co-digestion, without requiring the addition of magnetic nanoparticles during the anaerobic co-digestion process; it simplifies the process flow, achieves harmless treatment of microalgae and waste activated sludge, and further promotes methane production in the subsequent anaerobic co-digestion process; it also enables the recovery and reuse of magnetic nanoparticles, has simple operating equipment, and produces virtually no secondary pollution.

Description

Technical Field

[0001] This invention relates to the field of algae resource utilization and disposal, specifically to an integrated processing system that can harvest microalgae using recyclable magnetic nanoparticles while simultaneously promoting methane production. Background Technology

[0002] With the accelerating pace of urbanization, large amounts of nutrients such as nitrogen and phosphorus enter lakes, leading to increasingly severe eutrophication. This results in the proliferation of algae and large-scale algal blooms, a problem faced by many famous freshwater lakes in my country, such as Dianchi, Taihu, and Chaohu. To address this issue, researchers have developed numerous control measures, such as algae removal, flocculation, and biological algae control. These measures have effectively suppressed further algal blooms. However, if the removed or killed algae are not treated and disposed of scientifically and effectively, they will inevitably cause secondary pollution to the lake water or surrounding environment, necessitating harmless and reduced-volume treatment. Furthermore, since algae themselves contain large amounts of organic matter such as proteins and sugars, transforming them from a harmful substance into a beneficial resource is of great practical significance for solving current environmental and energy problems and achieving comprehensive governance goals.

[0003] Besides biomass yield and engineering operations, dehydration and harvesting are the main challenges in microalgae biomass production. However, microalgae harvesting is a crucial step in biofuel production, accounting for 20-30% of the total cost of biomass processing, and this high cost limits the development of the microalgae biofuel industry. This is partly due to (1) the small size of microalgae, with diameters generally ranging from 2-20 μm and densities similar to water, resulting in slow settling speeds and difficulties in cell separation; (2) the negatively charged surface of microalgae cells across a wide pH range, causing mutual repulsion between cells, resulting in stable suspension in the liquid and making aggregation difficult; and (3) the large volume of liquid containing microalgae required for processing. Especially after industrialization, large processing volumes will become an important consideration. Currently, there are various mechanical and chemical methods for microalgae harvesting, mainly including flocculation, centrifugation, filtration and screening, gravity sedimentation, flotation, and electrophoresis. Although these methods have achieved good performance and high harvesting efficiency, they are generally very expensive and energy-intensive (e.g., filtration and centrifugation). Compared to centrifugation, chemical coagulation and flocculation can further enhance gravity sedimentation and dissolved air flotation, with low energy consumption. However, flocculants are expensive and prone to causing secondary pollution. Furthermore, subsequent sedimentation takes a long time, resulting in low harvesting efficiency. Therefore, magnetic separation has attracted increasing attention from researchers as a promising alternative. Magnetic flocculation, by adding magnetic materials, utilizes unique superparamagnetism and bioaffinity to allow magnetic nanoparticles to quickly and effectively capture microalgae, which are then harvested using an external magnetic field. Magnetic flocculation offers advantages such as high harvesting efficiency, short harvesting time, and virtually no secondary pollution. Moreover, its ease of manipulation and regeneration, use of simple equipment, and cost-effectiveness make it a promising industrial application for microalgae harvesting.

[0004] Anaerobic digestion is a microbial-mediated process widely used in wastewater treatment, the disposal of organic household waste, and solid waste from agriculture, forestry, and animal husbandry. It converts complex organic waste into renewable energy in the form of biogas (primarily a mixture of methane and carbon dioxide). Microalgae contain abundant usable organic matter such as lipids, sugars, and proteins. Among various biofuel production methods, methane production from algal biomass is considered the most efficient in terms of energy recovery and utilization. This is because not all triglycerides can be transesterified into biodiesel, nor can all sugars be fermented to produce bioethanol. Instead, these three macromolecules (lipids, carbohydrates, and proteins) can be used as precursors by anaerobic microorganisms to produce biogas. Anaerobic treatment can reduce sludge volume, lower the cost of final sludge disposal, inactivate pathogens and other microorganisms in the sludge to render it harmless, and recover large quantities of high-value-added products such as short-chain fatty acids, hydrogen, and methane from the sludge, achieving a "waste-to-resource" effect and playing a significant role in alleviating the current energy crisis.

[0005] Nanotechnology is an emerging technology for improving the performance of anaerobic digestion. Nanoparticles (1-100 nm) possess excellent physicochemical properties, such as high activity, high reactive surface area, chemical stability, high specificity for performance enhancement, and the ability to stimulate microbial growth during anaerobic digestion. The addition of nanoparticles affects the microbial community and, at appropriate concentrations, increases biomass degradation through direct or indirect interspecies electron transfer, thereby increasing biogas production. Among nanoparticles, iron-based nanoparticles appear to be the most promising nanomaterials for improving biogas production, enhancing the stability of biodigester processes, achieving better substrate treatment, and increasing pathogen reduction. Magnetic iron oxide nanoparticles, in particular, have gained widespread application in recent years due to their magnetic properties, non-toxicity, high coercivity, biocompatibility, and advantages such as improved electron transport efficiency, increased enzyme activity during methanogenesis, provision of nutrients for microorganisms, and reduction of the inhibitory effect of sulfate-reducing bacteria. In fact, iron ions (Fe... 2+ and Fe 3+ ) are essential components of cofactors and enzymes, and adding them to anaerobic digesters can enhance the activity of methanogenic archaea. Furthermore, the application of nanoparticles in improving anaerobic treatment performance has been limited by their susceptibility to loss, but magnetic iron oxide nanoparticles can overcome these drawbacks because they are easily recycled.

[0006] In summary, magnetic iron oxide nanoparticles can both harvest microalgae through magnetic separation and increase methane production through anaerobic digestion. However, to date, no studies have demonstrated the multifunctional role of magnetic iron oxide nanoparticles in integrated microalgae separation and anaerobic digestion systems. Summary of the Invention

[0007] In view of this, the present invention provides an integrated processing system for harvesting microalgae with magnetic nanoparticles while simultaneously promoting methane production. This system makes the processing of microalgae biofuels cost-effective, minimizes waste, and further promotes methane production in subsequent anaerobic co-digestion processes, demonstrating strong practicality.

[0008] To achieve the above objectives, the technical method employed in this invention is as follows:

[0009] Methods for preparing magnetic nanoparticles:

[0010] (1) Dissolve FeCl3 and FeCl2 in 100 mL of deoxygenated ultrapure water at a molar ratio of 2:1;

[0011] (2) Heat the suspension obtained in step (1) to 60°C and incubate it in a nitrogen atmosphere for 1 hour;

[0012] (3) Slowly add 0.5M NaOH solution to the suspension obtained in step (2) under nitrogen protection and stir until the pH value of the suspension is 9.0;

[0013] (4) The suspension containing magnetic iron oxide nanoparticles obtained in step (3) is continuously stirred at 60°C and under nitrogen for 1 hour. Then, the magnetic iron oxide nanoparticles are separated by neodymium permanent magnets, washed three times with ultrapure water, washed with ethanol, and dried in a vacuum dryer at 60°C for 12 hours to obtain the recyclable magnetic nanoparticles.

[0014] The integrated processing system that uses magnetic nanoparticles to harvest microalgae while simultaneously promoting methane production includes the following steps:

[0015] (5) Add the algal liquid and high-concentration magnetic iron oxide nanoparticle suspension from the storage tank to the mixing reactor in a certain proportion through the feed inlet using a metering pump, and start the stirring device.

[0016] (6) The magnetic-algae mixture is continuously fed to the magnetic separation reactor at a certain flow rate. The microalgae magnetic separation process is completed by the electromagnet at the bottom. The supernatant after separation flows into the effluent storage tank through the effluent outlet at the top of the reactor. During this period, the microalgae contained in the effluent are kept at a low concentration. The magnetic-algae aggregates after separation accumulate at the bottom of the reactor and are transported to the anaerobic co-digestion reactor through the discharge port below.

[0017] (7) In the anaerobic co-digestion reactor, magnetic-algae aggregates and waste activated sludge are added in a certain proportion by metering pump. The mass ratio between magnetic iron oxide nanoparticles and anaerobic digestion substrates (microalgae and waste activated sludge) is adjusted. Inoculum is added and water is added to make up to 80% (v / v) of the effective volume of the reaction vessel for all materials in the entire anaerobic digester. The initial pH is adjusted, and then nitrogen is blown off. The anaerobic digester is sealed and the reaction temperature is controlled. During the digestion process, the materials are stirred to make the reaction uniform. The residual mixture after digestion in the reactor is transported to the magnetic iron oxide nanoparticle separation and recovery unit through the discharge port below.

[0018] (8) Add a clear liquid without magnetic nanoparticles to the magnetic nanoparticle separation and recovery reactor until it comes into contact with the electromagnet above the reactor. Then, introduce gas from the gas inlet below the reactor and add the residual mixture after anaerobic co-digestion. The generated bubbles carry the magnetic iron oxide nanoparticles during the upward movement and promote their upward movement. Finally, the magnetic iron oxide nanoparticles are efficiently recovered by the electromagnet set above the reactor.

[0019] The bare core magnetic nano iron oxide particles described in step (4) above are dissolved in distilled water and ultrasonically treated for 15 to 30 minutes to completely disperse them, resulting in a uniformly dispersed suspension.

[0020] In step (4) above, the bare-core magnetic iron oxide nanoparticles and algal liquid are stored in a magnetic nanoparticle storage tank and an algal liquid storage tank, respectively. The magnetic nanoparticle storage tank is made of non-ferromagnetic materials such as plastic or glass.

[0021] In step (5) above, the algal solution is first added to the mixing reactor, and then high-concentration magnetic iron oxide nanoparticles are added. The dosage is controlled by a metering pump so that the mass ratio of the magnetic-algae mixture is 0.4 to 0.8.

[0022] The magnetic-algae mixing in step (5) above is completed by a built-in stirring device, with the stirring intensity set to 100-150 rpm / min and the stirring time to 10-20 min; the mixing reactor is made of non-ferromagnetic material.

[0023] The main body of the magnetic separation unit mentioned in step (6) above is a frustum-shaped reactor made of non-ferromagnetic material, wider at the top and narrower at the bottom. The magnetic-algae mixture inlet is located at 1 / 3 to 2 / 3 of the magnetic separation reactor, and the flow rate of the magnetic-algae mixture is 10 to 30 m³ / h. 3 / h.

[0024] The electromagnetic field strength of the electromagnet mentioned in step (6) above is 800-1600 Gs, and the microalgae content in the effluent after separation is less than 5% of the original algae liquid, that is, the harvest efficiency of microalgae magnetic separation is >95%.

[0025] The total suspended solids of the waste activated sludge in step (7) above are maintained in the range of 20-30 g / L, the mass ratio of the magnetic-algae mixture to the waste activated sludge is 1:6-10, and the added water is the effluent from the magnetic separation reactor's effluent storage tank.

[0026] In the anaerobic co-digestion described in step (7) above, the pH is adjusted to 7.0±0.1, the nitrogen used for nitrogen stripping has a purity of 99.5% or higher, the reactor temperature is 25-45℃, the stirring is carried out by a folding blade turbine stirrer, the stirring intensity is set to 100-200 rpm / min, and the digestion time is 9-18 days; the reactor described in step (7) is made of non-ferromagnetic material.

[0027] The magnetic nanoparticle separation and recovery reactor mentioned in step (8) above is made of non-ferromagnetic material, and the magnetic field strength of the electromagnet is 1000-3000 Gs.

[0028] The gas mentioned in step (8) above is selected from air or inert gas, and is blown upward from the bottom of the reactor by a blower through a microporous gas distribution pipe, with a gas flow rate of 300-1800 m³ / h. 3 / h.

[0029] The innovations and advantages of this study are as follows:

[0030] (1) Compared with the prior art, the present invention achieves a highly efficient integrated processing of microalgae harvesting and anaerobic co-digestion. Under the action of an external magnetic field, the microalgae harvesting rate can reach more than 95% while greatly shortening the harvesting time. In addition, the integrated processing flow of microalgae magnetic separation and anaerobic co-digestion eliminates the need to add magnetic iron oxide nanoparticles during the anaerobic co-digestion process, simplifying the process flow and having significant implications for the large-scale application of microalgae harvesting and methane production.

[0031] (2) This invention achieves the harmless treatment of microalgae and waste activated sludge, and while maximizing the reduction of solid waste, it can further promote methane production in the subsequent anaerobic co-digestion process. This integrated treatment system not only reduces the harm to the environment and even the human body, but also promotes the safe and effective application of anaerobic treatment technology, which is of great significance for the reduction and resource utilization of microalgae and sludge.

[0032] (3) This invention realizes the recovery and reuse of magnetic iron oxide nanoparticles. The operation equipment is simple and there is basically no secondary pollution. It makes the processing of microalgae biofuels cost-effective and has a promising industrialization prospect in the field of microalgae resource utilization. Attached Figure Description

[0033] Figure 1 This is a process flow diagram of the present invention. Detailed Implementation

[0034] The present invention will be further described below with reference to specific practical examples, but this does not limit the scope of protection of the present invention.

[0035] Implementation Case 1

[0036] (1) Magnetic nanoparticle uniform suspension: The synthesized magnetic iron oxide nanoparticles were uniformly dispersed in deionized water by ultrasonic treatment for 15 min to prepare a 200 g / L magnetic iron oxide nanoparticle uniform suspension, which was stored in a storage tank.

[0037] An integrated processing system that harvests microalgae using magnetic nanoparticles while simultaneously promoting methane production:

[0038] (2) In the plexiglass mixing reactor, 60% of the working volume of algal solution was pumped into the algal solution storage tank, and then high concentration of exposed magnetic iron oxide nanoparticles were pumped in to mix, so that the mass ratio of magnetic-algae mixture was 0.6; the mixture was stirred for 10 min with a stirring intensity of 120 rpm / min.

[0039] (3) In the plexiglass magnetic separation reactor, at 10m 3The magnetic-algae mixture is continuously pumped into the reactor at a flow rate of / h, and magnetic separation is performed by an electromagnet with a magnetic field strength of 1600Gs at the bottom. The microalgae content and particle content of the supernatant at the top of the reactor are sampled from the outlet. The algae recovery rate is 96% and the particle loss rate is 1%. The magnetic-algae aggregates enter the anaerobic treatment unit through the discharge port at the bottom of the reactor.

[0040] (4) Magnetic-algae aggregates obtained by magnetic separation were added to an anaerobic reactor made of plexiglass, followed by waste activated sludge and inoculum from a municipal wastewater treatment plant. The total suspended solids of the concentrated sludge (i.e., natural sedimentation at room temperature followed by removal of the supernatant) was 21.8 g / L. The mass ratio of the magnetic-algae mixture to the waste activated sludge was adjusted to 1:8. Water from the effluent tank was used to adjust the volume of the digestion substrate to 80% of the container volume. The pH of the anaerobic digestion substrate was adjusted to 7.0 ± 0.1 using sodium hydroxide solution. Nitrogen gas with a purity of 99.5% or higher was introduced into the reactor and carried out for 5 minutes to remove oxygen. The reactor was then sealed for anaerobic treatment. The reactor temperature was 35 ± 1 °C. Stirring was performed using a folding blade turbine stirrer with a stirring intensity of 120 rpm / min. The digestion time was 16 days, and the highest methane yield was 232.3 ml / g VS.

[0041] (5) Add a clear liquid without magnetic nanoparticles to the organic glass magnetic nanoparticle separation and recovery reactor until it comes into contact with an electromagnet with a magnetic field strength of 2000 Gs above the reactor; blow air upwards from the bottom of the reactor through a microporous air distribution pipe using a blower, with a gas flow rate of 900 m³ / s. 3 / h; then add the residue after anaerobic co-digestion, and adsorb magnetic nanoparticles by adjusting the electromagnet above the reactor to always be in contact with the liquid surface, with a recovery efficiency of 90%.

[0042] Implementation Case 2

[0043] (1) Magnetic nanoparticle uniform suspension: The synthesized magnetic iron oxide nanoparticles were uniformly dispersed in deionized water by ultrasonic treatment for 20 min to prepare a 300 g / L magnetic iron oxide nanoparticle uniform suspension, which was stored in a storage tank.

[0044] An integrated processing system that harvests microalgae using magnetic nanoparticles while simultaneously promoting methane production:

[0045] (2) In the plexiglass mixing reactor, 60% of the working volume of algal solution was pumped into the algal solution storage tank, and then high concentration of exposed magnetic iron oxide nanoparticles were pumped in to mix, so that the mass ratio of magnetic-algae mixture was 0.4; the mixture was stirred for 15 min with a stirring intensity of 150 rpm / min.

[0046] (3) In the plexiglass magnetic separation reactor, at 15m 3The magnetic-algae mixture is continuously pumped into the reactor at a flow rate of / h, and magnetic separation is performed by an electromagnet with a magnetic field strength of 1600Gs at the bottom. The microalgae content and particle content of the supernatant at the top of the reactor are sampled from the outlet. The algae recovery rate is 95% and the particle loss rate is 1%. The magnetic-algae aggregates enter the anaerobic treatment unit through the discharge port at the bottom of the reactor.

[0047] (4) Magnetic-algae aggregates obtained by magnetic separation were added to an anaerobic reactor made of plexiglass, followed by waste activated sludge and inoculum from a municipal wastewater treatment plant. The total suspended solids of the concentrated sludge (i.e., natural sedimentation at room temperature followed by removal of the supernatant) was 26.3 g / L. The mass ratio of the magnetic-algae mixture to the waste activated sludge was adjusted to 1:6. Water from the effluent tank was used to adjust the volume of the digestion substrate to 80% of the container volume. The pH of the anaerobic digestion substrate was adjusted to 7.0 ± 0.1 using sodium hydroxide solution. Nitrogen gas with a purity of 99.5% or higher was introduced into the reactor and carried out for 5 minutes to remove oxygen. The reactor was then sealed for anaerobic treatment. The reactor temperature was 30 ± 1 °C. Stirring was performed using a folding blade turbine stirrer with a stirring intensity of 120 rpm / min. The digestion time was 16 days, and the highest methane yield was 276.4 ml / g VS.

[0048] (5) Add a clear liquid without magnetic nanoparticles to the organic glass magnetic nanoparticle separation and recovery reactor until it comes into contact with an electromagnet with a magnetic field strength of 2500 Gs above the reactor; blow air upwards from the bottom of the reactor through a microporous air distribution pipe using a blower, with a gas flow rate of 800 m³ / s. 3 / h; then add the residue after anaerobic co-digestion, and adsorb magnetic nanoparticles by adjusting the electromagnet above the reactor to always be in contact with the liquid surface, with a recovery efficiency of 90%.

Claims

1. An integrated processing system for recycling magnetic nanoparticle-harvested microalgae while promoting methane production, comprising, in sequence, a bare-core magnetic nanoparticle preparation unit, a microalgae mixing and magnetic separation unit, an anaerobic co-digestion unit, and a magnetic particle recycling and reuse unit, characterized in that, The method comprises the following steps: S1, preparing bare core magnetic nanoparticles; the bare core magnetic nanoparticles are prepared by the following method: S1-1, the bare core magnetic nanoparticle preparation unit is used for mixing FeCl3 and FeCl2 in a molar ratio of 2:1 in 100 mL of deoxygenated ultrapure water to obtain a suspension under nitrogen protection, and reacting at 60 DEG C for 1 hour; S1-2, under nitrogen protection, the pH of the suspension is adjusted to 9.0 with a 0.5 mol / L NaOH solution, and stirring is continued at 60 DEG C for 1 hour; S1-3, the bare core magnetic nanoparticles are separated by a magnet with a magnetic field strength of 800-1600Gs, washed three times with ultrapure water and once with ethanol, dried at 60 DEG C for 12 hours, and then ultrasonically dispersed in deionized water for 15-30 minutes to obtain a uniform bare core magnetic nanoparticle suspension. S2, harvesting microalgae while promoting methane production using bare core magnetic nanoparticles, comprising the following steps: S2-1, the microalgae mixing and magnetic separation unit is used for mixing the bare core magnetic nanoparticles with the microalgae suspension at a mass ratio of 0.4-0.8, stirring at 100-150 rpm for 10-20 minutes, and separating by a magnetic separator with a magnetic field strength of 800-1600Gs and a flow rate of 10-30 m3 / h, with an algal body recovery rate of ≥95%; S2-2, the anaerobic co-digestion unit is used for mixing the magnetic-algae aggregates obtained by magnetic separation with the remaining activated sludge at a mass ratio of 1:6-10, adding inoculum, and stirring at 100-200 rpm under the conditions of 25-45 DEG C and pH = 7.0 ± 0.1 for 9-18 days after nitrogen stripping; S2-3, the bare core magnetic nanoparticle recycling and reuse unit comprises an electromagnetic recycling device, and a combination process of air floatation method and electromagnetic recycling is adopted, the air blowing flow rate is 300-1800 m 3 / h, the magnetic field strength is 1000-3000 Gs, the bare core magnetic nanoparticle recycling rate is greater than or equal to 90%, and the recycled bare core magnetic nanoparticles are repeatedly used in the microalgae mixing and magnetic separation unit.

2. The integrated system for harvesting microalgae and promoting methane production from recycled magnetic nanoparticles according to claim 1, wherein, The main body of the magnetic separation unit in step S2-1 is a circular truncated cone type non-ferromagnetic material reactor with a wide upper part and a narrow lower part, and the magnetic-algae mixture inlet is located at 1 / 3-2 / 3 of the magnetic separation reactor.

3. The integrated system for harvesting microalgae and promoting methane production from recycled magnetic nanoparticles according to claim 1, wherein, In step S2-2, the total suspended solids of the waste activated sludge are maintained in the range of 20-30 g / L; in step S2-2, the nitrogen gas used for nitrogen stripping has a purity of more than 99.5%, the stirring is performed by a folding turbine stirrer, and the reactor is made of non-ferromagnetic material.

4. The integrated system for microalgae harvesting and methane production using recyclable magnetic nanoparticles according to claim 1, wherein, In step S2-3, the gas is selected from air or inert gas, and is blown from the bottom of the reactor to the top through a microporous gas distribution pipeline by a fan, the generated bubbles carry the bare core magnetic nanoparticles in the digestion residues upward during the floating process, and finally the bare core magnetic nanoparticles are efficiently recovered by an electromagnet arranged above the reactor.

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